1. Field of the Invention
The present invention relates to an intermediate transfer device for use in electrophotography, and more particularly, an image forming apparatus, such as a copier, a facsimile, a printer, or a multifunctional peripheral, using the intermediate transfer device.
2. Description of the Background
In a typical image forming apparatus, a toner image is fixed on a recording medium, such as paper, by application of heat and pressure from a fixing device. Since the surface of the recording medium has microscopic concavities and convexities due to the presence of fibers, the toner image deforms along the microscopic concavities and convexities when fixed thereon. The degree of deformation of the toner image varies by location on the surface, but the toner image on convex portions of the surface largely deforms.
For the above reason, a toner image fixed on a recording medium having a rough surface can appear grainy. In particular, a toner image formed on a concave portion deforms much less when fixed thereon because the toner image is not in contact with a pressing member or a pressing force applied to the toner image from the pressing member is small. Therefore, there is a difference in surface texture between toner images fixed on convex portions and concave portions of the surface of the recording medium, resulting in uneven gloss in the fixed toner image.
To solve the above-described problem, various image forming apparatuses employing an intermediate transfer member have been proposed.
Japanese Patent No. (hereinafter JP) 3528371 discloses an image forming apparatus including an endless-belt-like image bearing member, a rotatable endless-belt-like intermediate transfer member in contact with the image bearing member, and a heating roller configured to heat a toner image transferred onto the intermediate transfer member to a temperature higher than the melting temperature of the toner. The image forming apparatus further includes a support roller configured to support the intermediate transfer member and a pressing roller configured to press the intermediate transfer member with a transfer member therebetween, both provided on a downstream side from the heating roller relative to a rotation direction of the intermediate transfer member, so that the toner image on the intermediate transfer member is transfixed on the transfer member.
However, there is still a problem in that the toner image deforms to a greater or lesser extent when fixed on the transfer member. This is because the toner image has been previously deformed on the intermediate transfer member when heated to a temperature higher than the melting temperature of the toner, and furthermore, as noted above, the toner image is pressed against microscopic concavities and convexities formed on the surface of the transfer member due to the presence of fibers at a nip formed between the inter mediate transfer member and the pressing roller. A toner image fixed on a convex portion largely deforms, in other words, a toner image fixed on a convex portion has a relatively large area. By contrast, a toner image fixed on a concave portion deforms less, in other words, a toner image fixed on a concave portion has a relatively small area. As a result, low-quality images of uneven granularity and glossiness are produced.
Unexamined Japanese Patent Application Publication No. (hereinafter JP-A) 2001-13798 discloses an image forming apparatus in which a toner image formed on an image bearing member is transferred onto an intermediate transfer member, wherein a plane heater provided on an inner side of the intermediate transfer member heats the toner image with the intermediate transfer member therebetween, so that at least part of toner particles included in the toner image are softened and melted so as to be integrated before the toner image passes through a nip at which a pressing roller presses a heating roller with the intermediate transfer member therebetween. The integrated toner image is then transfixed onto a recording medium at the nip.
Since the toner image has been previously heated and deformed, the toner image has a low viscosity, and therefore the fixed toner image on a recording medium has a height of from 40 to 80% of the unfixed toner image. Consequently, the area of the fixed toner image varies depending on microscopic concavities and convexities formed on the surface of the recording medium, resulting in poor image quality.
JP-A2005-266304 discloses a transfixing device including an image bearing member to bear a toner image and an integrating means for integrating toner particles included in the toner image, wherein an unfixed integrated toner image is transfixed onto a recording medium at a transfixing area. The transfixing device further includes an adhering means for giving an adherence property to a surface of the integrated toner image which contacts the recording medium, provided on an upstream side from the transfixing area relative to a conveyance direction of the recording medium. The adhering means provides adherence by using radiant heat, applying a fixing auxiliary agent, etc.
In particular, the toner image on an intermediate transfer member is melted by heating a toner integrating roller provided facing the intermediate transfer member. Since the toner image is more likely to adhere to a member having a higher temperature, there is a problem in that the toner image easily adheres to the heated toner integrating roller.
FIG. 1 is a schematic view illustrating a process of deformation of a toner image T in a background-art intermediate transfer device 200.
FIG. 2 is a schematic view for explaining deterioration of the toner image T, transferred using the background-art intermediate transfer device 200, when fixed on a recording medium P having a rough surface.
The toner image T is required to have a desired image density after being fixed on the recording medium P. As the surface roughness of the recording medium P increases, the toner image T needs to include a larger amount of toner particles to obtain the desired image density. This is because toner particles included in the toner image T tend to get into concave portions of the surface of the recording medium P when transferred thereon. For example, a 100% solid image may have a smaller toner area ratio, which is a ratio of an area to which toner particles are adhered, after transferred onto the recording medium P.
When the toner image T is transferred onto the recording medium P having a rough surface by the background-art fixing device 200 and fixed thereon by a fixing device, the toner area ratio of the toner image T slightly increases after being fixed, as illustrated in FIGS. 1(a) and 1(b). However, only toner particles transferred onto convex portions of the recording medium P receive pressure from the fixing device to be deformed, as illustrated in FIG. 2, resulting in a smaller deformation of the toner image T. Therefore, the toner image T needs to include a larger amount of toner particles to obtain the desired image density on the recording medium P having a rough surface.
For example, when a 100% solid image including toner particles in an amount of 0.4 mg/cm2 is transferred onto a recording medium having a high smoothness (i.e., Rz=2 μm), with toner particles having an average particle diameter of 6 μm, the transferred image has a toner area ratio of 90%, while the transferred image has a toner area ratio of 80% when transferred onto a recording medium having a low smoothness (i.e., Rz=50 μm). When the transferred image is fixed on the above-described recording media each having a high and a low smoothness, the fixed solid image has a toner area ratio of 97% and 82%, respectively. The recording media having a low smoothness needs toner particles in an amount of 0.5 mg/cm2 to obtain a desired image density.
When the toner image T on an intermediate transfer member 201 is previously deformed before transferred onto the recording medium P, the toner image T transferred onto the recording medium P has a smooth surface that results from a smooth surface of the intermediate transfer member 201, as illustrated in FIG. 1(c). Although the transferred toner image T has a smooth surface, the toner image T fixed on the recording medium P has a rough surface, as illustrated in FIG. 1(d), resulting in uneven glossiness of the toner image.
An image forming apparatus mounting the above-described intermediate transfer device typically employs a heat fixing method in which a toner image is fixed on a recording medium by application of heat and pressure. The fixing performance of the heat fixing method largely depends on the nature of the surface of the recording medium. When the recording medium has a rough surface, the toner image needs to receive a higher temperature or a greater pressure at a transfixing area to be satisfactorily fixed on the recording medium.
As the temperature of the toner image increases, that of an intermediate transfer member also increases. Further, the heat may transfer to an image bearing member, and therefore the temperature of the image bearing member also increases. As a result, the temperatures of other devices and members provided adjacent to the image bearing member also increase. Consequently, a toner contained in a developing device or a cleaning device may be aggregated on or adhered to the device, which is undesirable. Moreover, when a greater pressure is applied at a transfixing area, the durability of the image bearing member may deteriorate.